Improved scan, reduce, aggregate

[jmhofer]

scan is currently restricted to accumulators of type Func2<T, T, T> where Func2<R, T, R> would be possible for initial values of type R. This pull request tries to fix this by generalizing the scan implementation. I had to restructure the scan operator a bit for this to stay typesafe. Imho it's more readable now, though.

This indirectly leads to generalized reduce, too.

Also, aggregate is added as an alias for reduce (see #20).

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[benjchristensen]

Thank you for working on this.

• Observable had not had any changes done to it to expose the new reduce or scan methods so that should probably be added to this pull request.
• What names should we use on the public API?

Rx.Net never used reduce, it uses aggregate. Others call it fold, accumulate etc (more here http://en.wikipedia.org/wiki/Fold_(higher-order_function))

Java 8 is using the reduce and collector names (http://download.java.net/lambda/b74/docs/api/java/util/stream/Stream.html)

What names should we use and what aliases if any should we include in Observable?

[jmhofer]

About Observable - hmm, seems that the changes got lost somehow... I'll go dig them out again.

And about the names to use: imho RxJava should have the Java 8 names as well as the orginial Rx.Net names.

[benjchristensen]

Agreed with your opinion on naming. Can you add those when you update the Observable and make sure the Javadocs of the methods are clear in linking to each other when they are aliases of each other?

[jmhofer]

I've had a look at the Java 8 stream operations. To me, it looks like there's no explicit scan equivalent in Java 8. Or am I missing something?

collect seems to be specifically made for mutable operations. I don't see where we would use that.

So, I would just alias reduce with aggregate, and scan stays unaliased, if that's okay.

[benjchristensen]

That works for me, we can always add more aliases if Java 8 adds a scan equivalent.

[benjchristensen]

I plan on doing a release tomorrow (about 12 hours from now) if you want to try and have this ready for that.

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[jmhofer]

As soon as this is in, my swing branch becomes finally ready for a merge. It offers a few simple observables created from button click, keyboard and mouse events. Looking forward to this. :)